Eggs of the ascidian Phallusia mammillata were voltage clamped (from -100 to +60 mV) and inseminated with a low or heavy sperm concentration. From inseminations with low sperm concentration (1 x 10(6) sp/ml), we found that fertilization currents occurred between -100 and +40 mV: they were always inward and displayed an analogous pattern whatever the clamped voltage. We established that the percentages of inseminated eggs that produced a fertilization current varied as a function of the clamped voltage. These percentages were not statistically different from 100% at clamped voltages between -100 and -30 mV, they decreased to 68 and 56% at clamped Vm of -10 and 0 mV, respectively, but were not statistically different from 0% at clamped Vm between +10 and +40 mV. We never obtained any egg electrical response at a clamped voltage of +50 mV. Almost all eggs (96%) which responded electrically were penetrated by one or several spermatozoa. These eggs were resuming meiosis (81 to 50%) at values of clamped Vm between -100 and 0 mV, respectively. At clamped Vm between +10 and +50 mV, the percentages of eggs resuming meiosis were not statistically different from 0. These results indicate that in P. mammillata eggs, the occurrence of an electrical response is voltage dependent and consequently that the initial depolarizing shift of the fertilization potential constitutes a fast block to polyspermy. However, in this species, the sperm penetration is not voltage dependent, since it occurred at clamped Vm from -100 to +40 mV. On the other hand, when eggs were clamped from -100 to +60 mV and inseminated with a heavy sperm concentration (2 x 10(7) sp/ml), the curves expressing, respectively, the percentages of eggs which responded electrically, the percentages of eggs which were penetrated by one or several spermatozoa, and the percentages of eggs resuming meiosis, as functions of the clamped Vm, were shifted by approximately 35 mV toward more positive voltages, compared to the corresponding curves obtained from eggs inseminated with a low sperm concentration. This last result means that the critical value of the membrane potential which characterizes the electrical block to polyspermy is dependent on the sperm concentrations used for inseminations.